1. Field of the Invention
The present invention is directed to an X-ray computed tomography apparatus with multi-spectra beam hardening correction.
2. Description of the Prior Art
The attenuation p that X-radiation generated by an X-ray source experiences in a transirradiated subject is measured in X-ray computed tomography. It is determined from the X-ray intensity I0 incident onto the subject and from the intensity I that is registered in a detector arranged in the beam path following the subject, according to the following equation:
p=xe2x88x921n(I/0)xe2x80x83xe2x80x83(1).
In the case of mono-energetic radiation, the following applies for a homogeneous subject with the attenuation coefficient xcexc and the transirradiated subject thickness d:
p=xcexcd xe2x80x83xe2x80x83(2).
The X-ray attenuation thus increases linearly with the subject thickness.
In fact, however, an X-ray tube emits polychromatic X-radiation with the energy distribution S(E). The attenuation is then calculated according to the following equation:
p=∫xc2x7∫xcexc(E)S(E)dEdxxe2x80x83xe2x80x83(3).
Even when the subject is homogeneous, the X-ray attenuation produced by the subject is thus no longer linearly dependent on the transirradiated subject thickness. Since xcexcE usually decreases toward higher energies, the xe2x80x9cenergy center of gravityxe2x80x9d shifts toward higher energies, namely all the more the greater the transirradiated subject thickness is. This effect is referred to as beam hardening.
In image reconstruction methods that are standard in CT technology, a linear change of the X-ray attenuation with the subject thickness is assumed for homogeneous subjects. The overall attenuation p of a beam on its path through a subject composed of partial subjects i with attenuation coefficient xcexci and thickness di then derives from:
p=xcexa3i(xcexcidi)xe2x80x83xe2x80x83(4)
The deviations from this assumption caused by the beam hardening lead to data inconsistencies and, thus, to image errors. Typical image errors caused by beam hardening are key artifacts in large, homogeneous subjects and line or bar artifacts in CT images with a high proportion of bone or contrast agent. Current correction methods often have the principal goal or eliminating key artifacts and stripe artifacts in subjects with high attenuation, for instance in shoulder and pelvis exposures. These corrections usually ensue with what is referred to as polynomial correction, whereby a corrected attenuation value pc is calculated from a detected measured attenuation value pM by insertion into a polynomial with predetermined coefficients an according to the following equation:
pc=xcexa3[n=0.1 . . . N](anPMn)xe2x80x83xe2x80x83(5).
The coefficients an are acquired, for example, by measuring the attenuation values of uniform absorbers (for example, Plexiglas(copyright) bars) given N different thicknesses.
It has been shown that improved correction methods are needed for the correction of locally limited bar and line artifacts as well as unsharp bone-tissue transitions as particularly occur given skull exposures (another known stripe artifact, for example, is what is referred to as the Hounsfield stripe between the petrous bones). An approach has thereby proven beneficial wherein the length of the xe2x80x9cbase materialxe2x80x9d that the X-ray beam leading to a measured value has traversed in the body of the patient under examination is individually estimated for each measured attenuation value. In medical examinations, bone substance and soft tissue or, respectively, water, which has spectral attenuation properties similar to soft tissue, are usually selected as base materials. A method referred to as the two-spectra method, for example, is known from the pertinent literature for estimating the base material lengths traversed by an X-ray beam. In this method, two measured values with respectively different spectral energy distribution of the X-ray, which is equivalent to a different average energy of the X-ray, are registered. Given known attenuation coefficients xcexcW(E1) and xcexcW(E2) of water at the average spectral energies E1 and E2 and xcexcK(E1) and xcexcK(E2) of bone at these average energies, the following, approximate estimate is possible for the measured attenuation values p(E1) and p(E2) obtained given there energies E1 and E2:
p(E1)=dWxc2x7xcexcW(E1)+dKxc2x7xcexcK(E1)xe2x80x83xe2x80x83(6a)
p(E2)=dWxc2x7xcexcW(E2)+dKxc2x7xcexcK(E2)xe2x80x83xe2x80x83(6b).
The water and bone lengths dW and dK can then be estimated from these equations.
Corrected measured values pc(E1) or, respectively, pc(E2) can now be respectively determined in the following way for the average spectral energies E1 and E2:
pc(E1)=p(E1)+fE1(dW,dK)xe2x80x83xe2x80x83(7a)
pc(E2)=p(E2)+fE2(dW,dK)xe2x80x83xe2x80x83(7b)
The correction values fE1 and fE2 are taken from tables that were determined in advance either computationally or empirically for the average spectral energies E1 and E2.
Further information about the above two-spectra method can be found, for example, in the following publications:
1) P. M. Joseph, R. D. Spiftal, Journal of Computer Assisted Tomography, 1978, Vol.2, p.100;
2) P. C. Johns, M. Yaffe, Medical Physics, 1982, Vol.9, p.231;
3) G. H. Glover, Medical Physics, 1982, VOl.9, p.860;
4) A. J. Coleman, M. Sinclair, Physics in Medicine and Biology, 1985, Vol.30, No.11, p.1251.
In order to register measured values at two different average energies in a conventional CT apparatus, two successive revolutions of the X-radiator around the patient must be implemented. In the second revolution, a different beam prefiltering or a different tube voltage is used compared to the first revolution. A disadvantage of such a procedure, however, is that the measured results can exhibit inconsistencies due to patient movement or contrast agent flow.
It is an object of the present invention to provide a computed tomography apparatus which avoids the aforementioned disadvantages of conventional computed tomography systems.
This object is achieved in accordance with the principles of the present invention in an X-ray computed tomography apparatus having an X-ray radiator with at least two spring foci between which the X-ray radiator is switched to alternatingly emit X-ray beams respectively from the foci, a radiation detector disposed in the X-ray beams, with an examination subject adapted to be disposed between the X-ray radiator and the radiation detector, the radiation detector having a number of detection channels, a beam filter arrangement disposed in said X-ray beams preceding said subject, the beam filter arrangement having regions of different filter characteristics respectively allocated to the foci to give the X-ray beams respectively different energies that the subject, and wherein the X-ray radiator irradiates a slice of the subject with the X-ray beams from a number of projection angles with the X-ray beams, after attenuation by the subject, being incident on detector channels of the radiation detector in a projection range. The radiation detector, for each detector channel in the projection range, generates at least two measured projection values for the respective X-ray beams at different energies from the foci. The measured projection values are supplied to an electronic and evaluation reconstruction unit connected to the radiation detector, which determines a beam hardening-corrected projection value for each of the measured projection values, and reconstructs a tomographic image of the slice of the examination subject using these corrected projection values.
The different regions of the filter arrangement achieve the aforementioned different energy-influencing filter characteristics by being of different filter material, or being of the same filter material but having respectively different thickness profiles in the regions.
The varying material or/and the varying material thickness of the beam filter arrangement make it possible to realize different average energies of the X-rays entering into the examination subject with a single beam filter arrangement without having to change the beam filter arrangement. In particular, the projection measured values for the various average energies can be registered in immediate chronological proximity to one another, so that falsifying influences on the projection measured values due to contrast agent flow and physical movements on the part of the patient area voided. All projection measured values can then be registered in one revolution of the radiator of the radiator-detector arrangement.
Since a multi-spectra correction with estimate of the base material lengths will not be required in all examination scenarios, it is recommended that the beam filter arrangement be interchangeably mounted at the radiator-detector arrangement in order to also keep the employability of the computer tomography apparatus opened for other correction techniques as well.
The beam filter arrangement can be held in a simple way at a diaphragm carrier arranged radiator-proximate that carries a diaphragm arrangement for beam shaping of the x-radiation emitted by the radiator.
The evaluation and reconstruction unit can be fashioned for determining an effective projection value by weighted summation from corrected projection values determined in allocation to respectively one of the detection channels and respectively allocated to one of the spring foci and for reconstructing the tomographic image upon employment of the effective projection values. In this way, the effect can be compensated that the projection measured values of a detection channel are registered at different spectra.
The spring focus mode, however, also allows for the possibility of realizing slice projections of enhanced sampling density by fashioning the evaluation and reconstruction unit for reconstructing the tomographic image for a plurality of projection channels per slice projection that is equal to a multiple of the plurality of detection channels lying within the projection region of the respective slice projection corresponding to the number of spring foci. The evaluation and reconstruction is fashioned for employing the corrected projection values determined in allocation to respectively one of the detection channels for all spring foci in the reconstruction of the tomographic image as corrected projection values of neighboring projection channels.
The inventive also can be advantageously utilized in computed tomography devices wherein the detector of the radiator-detector arrangement is implemented with a number of detector elements arranged in at least two lines lying above one another, an identical detection channel being allocated to their detector elements lying above one another in a respective column. In this case, the beam filter arrangementxe2x80x94in allocation to at least a sub-plurality of at least two detector elements of each column of detector elements lying within the projection region of a slice projectionxe2x80x94can respectively comprise a region of different filter material or/and different thickness profile of the filter material. The radiator-detector arrangement is then fashioned for supplying a respective projection measured value for each detector element from this sub-plurality of detector elements in allocation to each column of detector elements lying within the projection region of this slice projection.
In order to avoid subjectively perceived changes between tomographic images based on the employment of different energy spectra that are reconstructed from the projection measured values of successive lines of detector elements, the evaluation and reconstruction unit can be fashioned for determining an effective projection value by weighted summation from the corrected projection values determined in allocation to respectively one of the columns and respectively allocated to one of the detector elements from the decreased number of detector elements and of reconstructing the tomographic image using the effective projection values.